Load tree blobs with more than 50MB only from a single goroutine. Very
large tree blobs with for example 400 MB size can otherwise require
roughly 1GB * streamTreeParallelism memory.
This assigns an id to each tree root and then keeps track of how many
tree loads (i.e. trees referenced for the first time) are pending per
tree root. Once a tree root and its subtrees were fully processed there
are no more pending tree loads and the tree root is reported as
processed.
